1. Field of the Invention
The invention relates to catalytic converters for purifying exhaust gases, and more particularly to method for forming catalytic converters having non-round honeycomb substrates wherein the method results in minimal buckling of the converter's metal shell and uniform compressive forces being exerted on the encircling mat and the honeycomb substrate.
2. Description of the Related Art
As is well known, the purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is generally achieved by an exhaust gas purification system in which a ceramic element having a honeycomb cell structure acts a catalyst carrier. More precisely, this honeycomb cell structure is covered with a catalyst, that contains a precious metal which functions, in the presence of O.sub.2, to convert noxious components of the exhaust gas, such as HC and CO, to CO.sub.2 and H.sub.2 O. The honeycomb cell structure is housed within a gas-tight, sheet metal or cast-metal heat resistant housing or can or shell.
Honeycomb structures currently employed are typically comprised of a ceramic material such as cordierite; a brittle material exhibiting limited mechanical strength. For this reason, catalytic converters in use today, typically include a supporting mat that is wrapped around the periphery of the honeycomb. This resilient material, which distributes any compressive forces uniformly on the ceramic, typically expands as the temperature increases. This being the case, the compressive supporting pressure on the honeycomb therefore increases at elevated temperatures, and in some degree compensates for the thermal expansion of the outer metal shell. Since the metal shell expands more than the enclosed ceramic honeycomb, this mat expansion with temperature rise prevents the honeycomb from becoming loose in the shell.
There are known in the art various methods of fabricating catalytic converters as described above, including inserting tight-fitting mat-wrapped honeycombs into tubular shells (see, for example U.S. Pat. No. 4,093,423 (Neumann)), as well as utilizing two metal shell halves which are closed around a mat-wrapped honeycomb and thereafter welded together; see for example U.S. Pat. No. 5,273,724 (Bos). Another such method of fabrication, commonly referred to as the "tourniquet wrap" method involves forming a rectangular flat-sheet metal piece into a cylindrical body having a lap joint. A mat-wrapped honeycomb is loosely inserted into the cylindrical metal can and the combined assembly is pulled together to form the desired mat compression. Thereafter, the lap joint is welded together thereby holding the can at the desired compression while at the same time preventing gas leakage; see for Example U.S. Pat. No. 5,082,479 (Miller).
Although round substrates have some advantages in terms of uniform mounting and fundamental strength, the available space in an under-car and motorcycle applications has lead to the use of non-round shapes which are capable of providing sufficient catalyst surface area within the limited under-car and motorcycle space available. An inherent deficiency of the aforementioned formation techniques when used for non-round, oval or similar, shapes is uneven or non-uniform compressive closing of the encircling mat. Specifically, the mat portion located along the substrate's flatter side, i.e., along the minor axis, is less compressed than those rounder, smaller end portions of the substrate, i.e., along the major axis. On the one hand, the inadequate compression of the flatter sides results in an axial retention, i.e., the restraining forces which hold the substrate in place, which is decidedly lower than desirable and thus decreases product durability. On the other hand, the over-compressed small ends, areas where the mat gap is the small, lead to an increased risk of substrate failure due to point loading and localized compressive failure of the honeycomb structure, i.e., crushing of the brittle honeycomb structure.
This non-uniform compression problem has been addressed by various means including the use of deformed metal cans which provide less clearance along the flat sides, as well as the use of ribbing in the configuration which increases the rigidity of the can in the flatter areas.
A recent innovation disclosed in U.S. patent application Ser. No. 09/013,976 (Schmitt) discloses a method for manufacturing a catalytic converter having a non-round monolithic ceramic substrate. The first step of the method involves wrapping a non-round monolithic ceramic substrate in a sufficient amount of supporting mat material to substantially cover the peripheral surface of the substrate. Next, the wrapped substrate is thereafter inserted into a metal shell that substantially surrounds the wrapped substrate and at least one force redistribution plug is placed on the peripheral surface of the metal shell. Lastly, the metal shell is compressively closed around the substrate and the metal shell is secured to provide a gas tight seal and to hold the compressive stress.
This method generally results in a more uniform compression being exerted on the wrapped substrate, however, the metal shell, upon compression, is subject to unbalanced forces at the nodal points of these oval converters, i.e., points where the curvature of transitions from a small to large curvature. The result of these unbalanced forces is that the metal shell is typically subject to buckling along its periphery. Although this method alleviates the over/under compression problem somewhat, the method is unnecessarily complex, and is subject to the aforementioned buckling, and as such the search for better and simpler solutions to uniform oval canning has continued.